It is important in flat panel displays of the field emission cathode type that an evacuated cavity be maintained between the cathode electron emitting surface and its corresponding anode display face (also referred to as an anode, cathodoluminescent screen, display screen, faceplate, or display electrode).
There is a relatively high voltage differential (e.g., generally above 300 volts) between the cathode emitting surface (also referred to as base electrode, baseplate, emitter surface, cathode surface) and the display screen. It is important that catastrophic electrical breakdown between the electron emitting surface and the anode display face be prevented. At the same time, the narrow spacing between the plates is necessary to maintain the desired structural thinness and to obtain high image resolution.
The spacing also has to be uniformly narrow for consistent image resolution, and brightness, as well as to avoid display distortion, etc. Uneven spacing is much more likely to occur in a field emission cathode, matrix addressed flat vacuum type display than in some other display types because of the high pressure differential that exists between external atmospheric pressure and the pressure within the evacuated chamber between the baseplate and the faceplate. The pressure in the evacuated chamber is typically between about 10.sup.-4 and about 10.sup.-8 Torr.
Small area displays (e.g., those which are approximately 1" diagonal) normally do not require spacers, since glass having a thickness of approximately 0.040" cam support the atmospheric load without significant bowing, but as the display area increases, spacer supports become more important. For example, a screen having a diagonal measurement of 30" will have several tons of atmospheric force exerted upon it. As a result of this force, spacers will play an essential role in the structure of the large area, light weight, displays.
Spacers are incorporated between the display faceplate having a phosphor screen and the baseplate upon which the emitter tips are fabricated. The spacers, in conjunction with thin, lightweight, substrates support the atmospheric pressure, allowing the display area to be increased with little or no increase in substrate thickness.
Spacer structures must conform to certain parameters. The supports must 1) be sufficiently non-conductive to prevent catastrophic electrical breakdown between the cathode array and the anode, in spite of both the relatively close inter-electrode spacing (which may be on the order of 200 .mu.m), and relatively high inter-electrode voltage differential (which may be on the order of 300 or more volts); 2) exhibit mechanical strength such that they prevent the flat panel display from collapsing under atmospheric pressure; 3) exhibit stability under electron bombardment, since electrons will be generated at each of the pixels; 4) be capable of withstanding "bakeout" temperatures of around 400.degree. C. that are required to create the high vacuum between the faceplate and backplate of the display; and 5) be of small enough width so as to not visibly interfere with display operation.
There are several drawbacks to the current spacers and methods. Methods employing screen printing, stencil printing, or glass balls suffer from the inability to provide a spacer having a sufficiently high aspect ratio. The spacers formed by these methods are either too short to support the high voltages, or are too wide to avoid interfering with the display image.
Reactive ion etching (R.I.E.) and plasma etching of deposited materials suffer from slow throughput (i.e., time length of fabrication), slow etch rates, and etch mask degradation. Lithographically defined photoactive organic compounds result in the formation of spacers which are not compatible with the high vacuum conditions or elevated temperatures characteristic in the manufacture of field emission flat panel displays.
Accordingly, there is a need for a high aspect ratio space in an FED and an efficient method of making an FED with such a spacer.